1. Field of the Invention
The present invention relates to a surface emitting laser, particularly a vertical cavity surface emitting laser, and to an image forming apparatus using the surface emitting laser.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is one type of surface emitting laser. Because light can be emitted in a direction perpendicular to the principal plane of a semiconductor substrate in this type of laser, a two-dimensional array can be easily formed. Parallel processing of multiple beams emitted from the two-dimensional array can provide higher resolution and higher speed, and various industrial applications are expected to be achievable. For example, when a surface emitting laser array is used as an exposure light source of an electrophotographic printer, higher resolution printing and higher printing speed can be achieved through the use of parallel processing of multiple beams in a printing step. However, because minute spots are formed on a photoconductor drum with laser beams during electrophotographic printing, laser beams with a single transverse mode are required.
In recent years, a method for forming a current confinement structure by selectively oxidizing, for example, AlGaAs having an Al content of about 98% has been introduced for a surface emitting laser. This reduces the amount of unnecessary leakage-current and significantly improves light-emitting efficiency.
However, a selectively oxidized structure may not be appropriate in consideration of the single transverse mode. This is because oxidized layers cause a large refractive index difference and a higher order transverse mode may also exist stably. In particular, when a light-emitting area is enlarged to a diameter of 10 μm or more to achieve a higher output, even higher order transverse mode oscillation may occur.
Thus, single transverse mode oscillation is normally achieved by decreasing the diameter of the current confinement portion in the oxidized confinement structure to about 3 μm.
However, such a small diameter of the current confinement portion decreases the light-emitting area, which significantly reduces an output per element. Because current is injected into a minute light-emitting region, the resistance of an element is considerably increased. When current is injected into an element with higher resistance, the temperature increases and gain may decrease.
Some methods for achieving single transverse mode oscillation while maintaining a rather large light-emitting area, by intentionally introducing a loss difference between a fundamental transverse mode and a higher order transverse mode, have been considered.
One such method is to achieve single transverse mode oscillation by increasing the cavity length so as to increase diffraction loss of high order transverse modes, which is described in IEEE Photonics Technology Letters, Vol. 12, No. 8, 2000, p. 939. In this document, a long cavity structure is formed by disposing a GaAs layer with a thickness of 4 μm or more in a cavity to achieve a high-output single transverse mode in a surface emitting laser having a wavelength of 980 nm. This long cavity structure increases diffraction loss of high order transverse modes, and a single transverse mode can oscillate even in a relatively large light-emitting area (diameter of 7 μm).
However, the inventors of the present invention described herein found that the long cavity structure described in IEEE Photonics Technology Letters, Vol. 12, No. 8, 2000, p. 939, provides multi-longitudinal mode oscillation, which hardly occurs in known VCSELs.
In a one-wavelength cavity used in a surface emitting laser, a small cavity length of about 0.3 μm causes a longitudinal mode spacing of 50 nm or more, whereby a single longitudinal mode operation is easily achieved.
In a structure where a spacer layer having a thickness of 2 to 10 μm is inserted into a cavity, the longitudinal mode spacing decreases to about 10 nm. As the amount of current injection is increased to obtain a desired optical output, the gain peak shifts to longer wavelengths due to heat. As a result, the longitudinal mode hops to the next mode at longer wavelengths.
FIG. 11 shows examples of results from a surface emitting laser in which longitudinal mode hopping is demonstrated in an experiment conducted by the inventors. In this experiment, a spacer layer having a thickness of 2 μm was inserted into a cavity and a desired resonant wavelength was assumed to be 670 nm. As shown in FIG. 11, the surface emitting laser oscillates at the desired wavelength in cases where the amount of current injection is small, that is, 3 mA or less. However, the longitudinal mode hops to the next resonant mode of 685 nm in cases where the amount of current injection is increased to 4 mA or more to provide more optical output.
When the longitudinal mode hops in such a manner, emission intensity or a far-field pattern becomes unstable. For example, such longitudinal mode hopping is inappropriate for a light source of an apparatus that is required to stably form a beam spot, such as a photolithographic exposure apparatus.
In terms of parameters of crystal growth, including thickness control and surface roughness caused by an increase in cavity length, forming thick cladding layers having a thickness of several micrometers to achieve a long cavity structure should be avoided.